Multi Axis Vibration

What is Multi-Axis Vibration Testing?

Single Axis (Uniaxial) Vibration Shaker has been the common vibration testing system used for vibration test. Historically, this is the method which was firstly developed to perform vibration test and to bring in the end user environment into the laboratory scale test.

But the actual end user environment shows that the product delivered to customer is subjected to vibration on all directions simultaneously. In order to fulfill the requirement of applying the load as close as possible to the actual environmental load condition, the method of sequentially applying uniaxial excitation has been introduced, a multi axis vibration. This method is implemented by using a uniaxial shaker and applying the load along three orthogonal axis by rotating the test article after each test.

Multi axis vibration testing is a complex testing method whereby a sample is shaken or excited along more than one axis. Multi axis vibration testing is performed by applying a single-axis vibration to a sample along the X, Y and Z axes of the product. These tests are usually performed using a linear shaker and rotating the sample onto the following axis after each round.

Although the single axis shaker is still widely used for vibration testing, latest research has shown that there are some conditions which are not appropriate to be simulated by this equipment. Below is one simulation which has been done to show the difference between Single Axis Testing and Multi Axis Testing.

From above simulation results, we may see that Single Axis Testing and Multi Axis Testing will deliver different response. This different response will lead to different fatigue damage distribution and different failure mode as well.

Multi-axis vibration testing shakes the sample along multiple axes, and it is therefore a more complex test, both due to the number of elements involved, and due to the axes along which the shaking takes place. Naturally, these tests are much more effective when obtaining results that are closer to real-world conditions.

Types and function of multi-axis vibration tests

There are many ways to configure a multi-axis vibration test. The two key characteristics of any configuration are the number of shakers and the number of axes / degrees of freedom.

Let’s consider some examples.

A MESA (multi-exciter/single-axis) configuration is composed of two or more shakers that move in the same direction along one axis. This configuration is used to test oversized products that require a shaker on each end. The shakers are often synchronized and use the same test profile.

A four-post configuration has four shakers moving along the same axis. This method is often used for full vehicle testing in automotive and transportation industry. During such test, a shaker is placed under each wheel. Four recorded field data files from each wheel are played back. Then, the recorded vibrations are played back simultaneously as if the vehicle was in motion.

Mining vehicles operate in harsh outdoor conditions, factories and field mines. The special utility vehicle operating at high vibration magnitude are not only prone to high fatigue stress, but also harsh conditions affect the life of the vehicle and the health safety of the driver. The unloaded mining vehicles empty weigh 23.5 tons when fully loaded weigh 73.5 tons. In the past, such full vehicle tests were not possible by using electrodynamic shakers due to the capacity limit of the testing system. Other solutions using hydraulic shakers are complex and very costly which also involve other huge infrastructures costs.

As in all areas of the economy, to gradually improve product reliability, such large force vibration test system can be more readily deployed in the defence industry, railways, automotive, electronics, civil construction, aerospace, aviation, ships and other broad based application.

In recent years, the automotive industry has also witnessed a rising interest in other multi-axis shaker configurations. The three-axis configuration belongs to the MEMA (multi-exciter/multi-axis) class. This method involves at least three shakers and motions along the X, Y, and Z axes simultaneously.

Three-axis testing is primarily used for component or sub-system testing. It is accomplished by random vibration testing along each axis using identical or individualized test profiles. The three-axis configuration creates a more realistic test compared to traditional single-axis testing. As we mentioned previously, a vehicle experiences vibration from many directions simultaneously; the three-axis configuration accommodates the three linear directions of motion.

What factors or elements should be of concerns when implementing multi axis vibration

The advantages of multi-axis testing are clear. Multi-axis vibration testing brings products to fatigue failure faster and is more realistic than sequential, single-axis testing. However, there are several concerns with multi-axis configurations.

Coupling

Multi-axis configurations raise a number of concerns, including the coupling of multiple shakers to a table. Ideally, a shaker along one axis should be able to transfer its motion to a table without affecting the other axes of motion. When the coupling is set up properly, a shaker should move the table along one axis without causing significant motion in the other two axes.

Cross-axis motion occurs when excitation along one axis excites another axis or axes. An example would be a slip table moving side to side when it should only be moving forward and back. While we cannot completely eliminate cross-axis motion, proper coupling methods can minimize its effect.

A common coupling method employs hydrostatic bearings, which allow connections to pivot freely and permit the transmission of motion from shaker to table. Sliding bearings are another option. Whatever the coupling method, the goal is the same: to transmit motion without exciting the other axes and to let the table move as freely as possible when excitation is applied.

Resonances

Resonances are a significant concern in multi-axis testing. In cross-axis motion, resonances are more difficult to control because excitation along several axes can potentially excite a resonance along another axis or axes. The Sound & Vibration article, Defining the Global Error of a Multi-Axis Vibration Test (PDF), describes the potential for differences in excitation levels during multi-axis testing as well as ways to account for the differences. As mentioned in the article, the reference profiles must be designed so that resonances are minimized not just along one axis, but along several.

Accelerometers

Accelerometers also raise concerns in multi-axis testing. Transverse sensitivities produce more noticeable effects because the system is intentionally moving in all directions. In addition, accelerometer placement must be considered with respect to both the number of accelerometers and their location. Most tables or fixtures aren’t perfectly rigid; as such, accelerometer location matters. Multiple accelerometers are often used in conjunction with one another to determine motion in a particular degree of freedom. In such cases, the effects of their placement are magnified.

Even the design of the head expander—i.e. table—is affected by multi-axis configurations, as resonances must be minimized not just along one axis but several.

In order to prevent these issues, it is essential to count with reliable vibration systems in combination with staff that has the necessary experience in the field of multi-axis vibration testing.

According to MIL-STD-810G Method 527, below are the conditions which require Multi Axis Testing in order to be able to represent the actual vibration environment: